Light arrangement for vision systems

ABSTRACT

A lighting arrangement for use in vision systems and imaging applications. The arrangement illuminates the object to be imaged with a supplied light of a specified illumination. The intensity, hue, and directionality of the supplied light can be varied. The desired illumination is specified either by manually operable controls or via a programming interface. The arrangement has a light source which includes light-emitting diodes mounted in a housing. The source provides a constant light output over a long lifetime. A light sensor senses the total illumination on the object being imaged resulting from ambient light in addition to the supplied light, and a negative feedback circuit in the arrangement can adjust the level of the supplied light in response so as to maintain a constant total illumination on the object despite variations in the ambient lighting.

FIELD OF THE INVENTION

The present invention relates generally to lighting sources forphotographic imaging applications, and more particularly tocomputer-controlled lighting sources for vision systems used in testingof products, components, or sub-assemblies.

BACKGROUND OF THE INVENTION

In modern manufacturing environments, vision systems are often used totest products, components, or sub-assemblies in order to verify that theitem has been properly manufactured or assembled. A vision systemperforms, in an automated way, a visual inspection analogous to thatwhich would otherwise be done by a person. Among other benefits, thevision system can relieve a human operator of the tedium associated withsuch an inspection, thus freeing him to perform other functions, and canautomatically maintain quality records and analyze trends so as alertthe manufacturing personnel of actual or potential quality problems withthe manufactured item.

Vision systems typically utilize a video camera to capture and digitizeimages of the item being tested, and a computer connected to the videocamera to analyze the digitized images and compare them to what isexpected to be observed. For the vision system to operate properly—thatis, to correctly identify tested items as being either good or bad—eachitem being tested must be appropriately illuminated for the vision testbeing performed. If items are illuminated improperly, the vision systemmay incorrectly identify a good item as a bad one, and reject it.Incorrectly rejecting a significant number of good items results inincreased manufacturing costs which ultimately get passed on to theconsumer.

In order to minimize the number of items which are incorrectly rejectedby the vision system as non-complying, once the appropriate level ofillumination for a particular test is determined, the light sourceshould be capable of providing a certain constant light output (orintensity) for desired intervals over a long period of time.Furthermore, if a varying amount of ambient light can also illuminatethe item during testing, the light source should be capable of bothdetecting this ambient illumination and varying its light output so asto maintain the total illumination on the item at a constant level.

Certain vision applications require the light source to provide light ofdifferent intensities during different tests, or for testing differentitems. For example, two identical items supplied from different vendorsor produced from two different manufacturing runs may have differentreflectivity, requiring different illumination levels for the visionsystem to operate properly. Or, in one test the vision system may viewthe item through a protective wrapping that requires more illumination,and in another test without the wrapping that requires lessillumination.

Other vision applications require the light source to provide differentcolor lighting. For example, a first color light may be used to viewsurface features on a partially-transparent window of an item, while asecond color light is used to view the internal features of the itemthrough the window.

In addition, to view surface features of an item that project in adirection towards the video camera lens, it is advantageous for thelight source to provide side, rather than frontal, illumination.Providing side illumination to these surface features can generateshadows which are more easily detected by the vision system. Also, whenilluminating highly reflective items, side illumination may reducecertain reflections which can interfere with proper vision systemoperation.

Some previous light sources use halogen or incandescent lighting, whichoften are subject to fading over time. In addition, such light sourcestend to have relatively short lifetimes, requiring frequent replacementof the light sources. These light sources also tend to generateexcessive heat which often requires cooling so as not to affect the itembeing tested or other elements of the vision system, thus resulting inadded cost or complexity of the light source. While fluorescent lightsources tend not to produce excessive heat, fading over time is still aproblem. With any of the above light sources, filters are required inorder to provide different color light output. Such filters areexpensive, not easily changed, and provide only a limited number ofdiscrete light colors. Other previous vision system light sources uselight-emitting diodes of a certain color. Where the intensity of any ofthese light sources is selectable, it is typically done manually.

Accordingly, the need still exists for a light source that can produce aspecified light output of a specified color, can maintain a constantlight output for desired intervals over a long period of time, and canautomatically adjust the light output so as to maintain a constantillumination level on an object under a variety of ambient lightingsituations.

SUMMARY OF THE INVENTION

In a preferred embodiment, the present invention may be implemented as anovel lighting arrangement that produces a constant light output of aspecified intensity, and of a specified hue and direction, over a longlifetime, and can automatically adjust the light output in the presenceof ambient light in order to maintain a contain illumination on theobject being illuminated. Such a lighting arrangement can beadvantageously used in vision system applications in manufacturing andinspection, where consistent lighting of the objects to be tested orinspected under all ambient lighting conditions is essential to accurateoperation of the vision system and avoiding incorrect rejection ofconforming objects.

The lighting arrangement includes a light source which generates apredetermined light output in response to a light control signalprovided by a light controller which is electrically connected to thelight source. The light source is mounted in a housing which, duringoperation, is positioned near the object to be illuminated. The lightcontroller has an input connectable to the vision system so that thevision system can specify to the light controller the predeterminedlight output to be provided by the light source. The light source has aplurality of lighting elements mounted in the housing. The lightingelements are preferably each controllable independently. While thehousing can take a variety of shapes, a preferred embodiment is atoroidally-shaped housing where the lighting elements are arranged onone of the planar surfaces of the housing so as to evenly illuminate anobject positioned adjacent that planar surface. The housing may includea diffuser mounted between the lighting elements and the object. Atleast some of the lighting elements of the preferred embodiment arelight-emitting diodes. These diodes may all generate light ofessentially the same color or hue, or at least some of thelight-emitting diodes emit light of a different hue from at least someothers of the light-emitting diodes; in addition, by varying theintensity of light emitted from the different color diodes, the hue ofthe light output can be varied. Alternatively, light-emitting diodesthat emit light of a variable hue can be used in other embodiments. Theintensity of the light emitted by each light-emitting diode iscontrolled by pulse width modulation. All the diodes may be pulse-widthmodulated in the same way so as to produce light of the same intensity,or different pulse width modulation signal may be applied to differentlight-emitting diodes so that different diodes emit light of a differentintensity. By varying the intensity of light output based on theposition of the light-emitting diodes in the housing, the directionalityof the illumination on the object may be varied.

Another embodiment of the present invention is a lighting arrangementwhich provides a predetermined illumination on an object which isilluminated by a varying ambient light as well as light supplied fromthe light source. The lighting arrangement includes an illuminationsensor positioned near the object which detects the total illuminationon the object resulting from both the supplied light and the ambientlight. The sensor produces an illumination signal proportional to thetotal illumination, and this signal is electrically transmitted to thelight controller. A negative feedback circuit in the light controllervaries the light control signal in response to the illumination signalso as to maintain the predetermined illumination on the object. Thedesired level of predetermined illumination can be externally specifiedto the lighting arrangement, such as by a vision system, via aprogramming interface on the light controller. Alternatively, thepredetermined illumination level can be set by manual controls on thelighting arrangement. Typically, the light sensor is mounted in the samehousing containing the lighting elements.

Yet another embodiment of the present invention is a method forilluminating an object with a predetermined illumination. The methodincludes specifying the predetermined illumination desired, and applyinglight of an initial intensity level derived from the predeterminedillumination. The total illumination on the object resulting fromapplying the initial intensity level light is sensed, and then acorrected intensity level based on the initial intensity level set andthe total illumination sensed is determined. Then light of the correctedintensity level is reapplied so as to illuminate the object with thedesired illumination. Typically the object is also illuminated byambient light as well as the light supplied from the lightingarrangement, and so the total illumination sensed includes the ambientlight illumination on the object as well as the supplied lightillumination. Preferentially, in order to compensate for fluctuations inthe ambient light illumination, the total illumination is periodicallysensed, and a new corrected intensity level determined and reapplied, soas to maintain the predetermined illumination.

Other aspects and advantages of the present invention will becomeapparent from the following detailed description, taken in conjunctionwith the accompanying drawings, illustrating by way of example theprinciples of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of a vision system embodying thepresent invention.

FIG. 2 is a perspective view of a light source according to the presentinvention usable in the vision system of FIG. 1.

FIG. 3 is a schematic block diagram of the light source of FIG. 2illustrating directional light sourcing.

FIG. 4 is a planar view of a variable-hue light source according to thepresent invention.

FIG. 5 is a schematic block diagram of a lighting arrangement accordingto the present invention.

FIG. 6. is an electrical schematic diagram of the light source of FIG.2.

FIG. 7 is a flowchart of a method for illuminating an object with apredetermined illumination using the lighting arrangement of FIG. 5.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawings, and more particularly to FIG. 1, there isillustrated a schematic representation of a vision system 10 embodyingthe present invention. Such a system is capable of producing a constantlight output of a specified intensity, and of a specified hue anddirection, over a long lifetime, and can automatically adjust the lightoutput in the presence of ambient light in order to maintain a containillumination on the object being illuminated. While it is apparent tothose skilled in the art that the exact vision system components mayvary from system to system, the vision system 10 includes a video camera12 capable of capturing digitized images of an item under test 14 andtransferring the images to a vision system controller 16. The system 10includes a light source 18 for illuminating the item 14 while the videocamera 12 is capturing images. The light source 18 is mounted in ahousing 20 that is positionable in optical proximity to the item 14 suchthat light from the light source 18 illuminates the item 14 duringoperation of the video camera 12. The light source 18 generates apredetermined illumination in response to a light control signal 22generated by a light controller 24 in electrical communication with thelight source 18. The light controller 24 includes an input 26 forreceiving from the vision system controller 16 an illumination commandspecifying the predetermined illumination. The illumination command mayinclude such light source parameters as the light intensity, lightcolor, and directionality of the light with respect to the item 14 aswill be discussed subsequently in greater detail.

Considering now the light source 18 in further detail and with referenceto FIG. 2, the light source 18 includes a plurality of lighting elementsdisposed in the housing 20. The housing 20 can be physically organizedinto different shapes and configurations according to the presentinvention. One preferred embodiment of the housing 20 is a ring light30. The ring light 30 has a toroidally-shaped housing 32, with thelighting elements 36 a,b disposed around one of the planar surfaces 34of the toroid. The ring light 30 has an aperture through which the lensof the video camera 12 can be aimed to view the item 14 illuminated bythe light source 18. In some embodiments, the planar surface 34 may becovered by a diffuser 38 which diffuses the light emitted by thelighting elements 36 a,b so as to more evenly illuminate the item 14.The diffuser 38 also physically protects the lighting elements 36 a,b.

Considering now the lighting elements 36 a,b of the light source 18 infurther detail, in the preferred embodiment the lighting elements arelight-emitting diodes (LEDs). In comparison to halogen, incandescent, orfluorescent lighting elements, LEDs have a much longer lifetime and arenot subject to fading over that lifetime. LEDs also turn on and offfaster than do these other types of lighting elements. The intensity ofthe light emitted from the LEDs is preferably controlled by pulse widthmodulation by the light controller 24, where the duty cycle of the powersignal applied to each of the LEDs by the light controller 24 determinesthe intensity. A higher duty cycle generates a higher intensity light.The light controller 24 can typically supply a discrete number ofdifferent duty cycles, preferably 256, in order to provide a range ofpossible light intensities from the LEDs. The intensity of each LED maybe controlled independently of the intensity of other LEDs. As bestunderstood with reference to FIG. 3, this allows the directionality ofthe lighting on the item 14 generated by the ring light 30 to be varied.For example, the LEDs located in quadrant 44 a of ring light 30 may beturned on with a relatively high intensity, which those LEDs located inthe other three quadrants 44 b of ring light 30 may be turned off orilluminated at a much lower level. The incident lighting provided toitem 14 will be seen by the video camera 12 as directional light comingfrom the side of the image nearest to quadrant 44 a. If the visionsystem is required to determine the size or location of surface featureswhich are in the same direction as the axis of 46 of camera 12, such asa height h of item 14, the shadow 48 cast by the directional lightingcan facilitate this testing.

All the LEDs 36 a,b may emit light of essentially the same color (orhue), or different color LEDs may be using in a single ring light 30.For example, LEDs 36 a may be red, while LEDs 36 b may be green. Theintensity of red LEDs 36 a may be controlled by light controller 24independently of the intensity of green LEDs 36 b. For example, red LEDs36 a may be illuminated while green LEDs 36 b are turned off to providea red light, then green LEDs 36 b may be illuminated while red LEDs 36 aare turned off to provide a green light. If both the red and green LEDsare illuminated to provide equal light intensities, a yellow (or brown)light will result; the hue may be varied by varying the relativeintensities of the red and green LEDs. Even though the video camera 12may provide only black-and-white images to the vision system, in certainapplications different color lights facilitate the testing of an item14. For example, a first color light may be used to view surfacefeatures on a partially-transparent window of item 14, while a secondcolor light may be required to view internal features of the item 14through the window.

In another embodiment of the light source 18, and as best understoodwith reference to FIG. 4, the light source 18 may contain red 36 a,green 36 b, and blue 36 c LEDs to allow a range of differentillumination hues to be provided by the light source 18 by choosingdifferent intensities for each of these primary colors. Alternatively,variable-hue LEDs, each of which is capable of providing light in arange of hues, may be used for any of the LEDs 36.

A preferred embodiment of the present invention, as best understood withreference to FIG. 5, is a lighting arrangement 50 for providing apredetermined illumination on an item 14 where the item 14 may also beilluminated by ambient light 58. The lighting arrangement 50 includes alight source 18 positioned in optical proximity to the item 14 which,during operation, illuminates the item 14 with a supplied light 52 of acertain intensity in response to a light control signal 22 from a lightcontroller 24. The lighting arrangement 50 further includes anillumination sensor 54 also positioned in optical proximity to the item14 which, during operation, detects the total illumination 56 resultingfrom both the supplied light 52 and the ambient light 58. Theillumination sensor 54 is preferable a reflective sensor which detectslight reflected by the item 14 and other nearby objects illuminated bythe light source 18. The sensor 54 produces an illumination signal 60which is proportional to the total illumination on the item 14.Typically the light source 18 and illumination sensor 54 are mountedwithin a housing 20, with the sensor 54 located in the same planarsurface (such as the surface 34) in which the lighting elements of thesource 18 are located.

The lighting arrangement 50 also includes a light controller 24 which iselectrically connected to the illumination sensor 54 and the lightsource 18. As will be discussed subsequently in further detail, thelight controller 24 has a negative feedback circuit for producing thelight control signal 22 in response to the illumination signal 60 so asto maintain the predetermined illumination on the object. The lightingarrangement 50 further includes at least one mechanism to specify to thelight controller 24 the desired predetermined illumination, parameter ofwhich may include the intensity, hue or color, and directionality. In apreferred embodiment, the light controller 24 has manually operablecontrols 64, such as pushbuttons or rotary knobs, which electricallyspecify the predetermined illumination to the electronic circuitry withthe light controller 24. Alternatively, the light controller 24 has anprogramming interface 66 through which a computer or computer-controlleddevice (such as the vision system controller 16) can transmit electricalsignals corresponding to the predetermined illumination to the lightcontroller 24. Typical programming interfaces usable with the lightingarrangement 50 include serial ports such as RS-232C or USB (unifiedserial bus), and parallel ports such as a Centronics interface. Othermanually operable controls 64 and programming interfaces 66 known tothose skilled in the art are usable with the present invention forspecifying the predetermined illumination as well as those describedherein.

Considering now in further detail the operation of the light controller24, and as best understood with reference to FIGS. 2, 5 and 6, the lightcontroller 24 generates a light control signal which is transmitted tolight source 18 via light control signal bus 22. Typically bus 22contains a control line for each individual LED 36 a,b in the lightsource 18, and a common return line. The light control signal has anindependent pulse width modulated signal for each LED 36 a,b in thelight source 18. As previously explained, the pulse width modulationdetermines the intensity of the light emitted from LEDs 36 a,b. Also aspreviously explained, where LEDs 36 a produce light of a different colorthan LEDs 36 b, the hue of light produced from light source 18 can bevaried by sending different intensity signals to LEDs 36 a than thosesent to LEDs 36 b.

With regard to the use of the variable-hue LEDs 36 mentioned heretofore,these LEDs 36 typically contain red, blue, and green elements, eachelement having a separate pulse width modulation control line. Byindependently varying the pulse width modulation applied to each ofthese control lines, the overall hue produced by LEDs 36 can be variedas desired.

In some embodiments, individual LEDs 36 a,b are not turned off totallyduring operation, but rather their intensity is reduced to a minimumlevel, so that the operator can easily verify visually that the lightsource 18 is properly connected to the light controller 24 and that alllighting elements of the light source 18 are operating properly.

In a preferred embodiment, an illumination signal is transmitted fromillumination sensor 54 to light controller 24 via an illumination signalbus 60. The illumination sensor 54 preferentially consists of a singlephotodiode, but an array of photodiodes, or other light sensorarrangements known to those skilled in the art, may alternatively beused with the present invention. The illumination sensor 54 typicallyprovides a signal whose level is proportional to the total illumination56 sensed by the sensor 54, and can be calibrated appropriately todetermine the total illumination 56. If photodiodes with fast responsetimes are utilized for the sensor 54, the sensor output can be filteredby a low-pass filter 72 in order to remove unwanted effects, such as aresponse to the 60 hertz frequency of ambient fluorescent lighting, orto the pulse-width modulation frequency of the LEDs. Typically a 5 hertzfilter will remove such unwanted effects, but still allow the lightingarrangement 50 to respond properly to lower frequency changes in ambientlighting 58, such as might be caused by the operator leaning over andmomentarily blocking the ambient light 58 from the item 14.

A microcomputer 74 controls the operation of the light controller 24.The microcomputer 74 typically includes a memory (for example, an EERAM)for storing a control program, read/writeable memory (for example, anEERAM) for storing data while power is applied to the controller 24, andnon-volatile read/writeable memory (for example, NVRAM) for storing data(such as the most recent predefined illumination) when power is notapplied to the controller 24. The microcomputer 74 also typicallyincludes an analog input for receiving the filtered illumination signalfrom the low-pass filter 72, a digital input for receiving illuminationcommands from the programming interface 66, inputs for receivingillumination information from the manually operable controls 64, andoutputs for generating the light control signal for the light source 18.As will be discussed subsequently, in some embodiments the microcomputer74 also includes an input port for receiving special effects signals.The microcomputer 74 can alternatively be implemented as amicroprocessor and external memory and logic circuitry, a digital signalprocessor (DSP), a state machine, or other such control systems known tothose skilled in the art.

As best understood with reference to FIG. 7, the microcomputer 74performs a control method to produce and maintain a predeterminedillumination on the item 14. The method begins, at 90, with apredetermined illumination that is specified to the light controller 24via the programming interface 66 or the manually operable controls 64.The microcomputer 74 then applies to the item 14 supplied light 52 of aninitial intensity level derived from the predetermined illumination, byproducing a light control signal corresponding to the initialillumination and transmitting it to the light source 18 (at 92). Next,at 94, the total illumination 56 on the item 14 resulting from applyingthe initial intensity level light is sensed by the illumination sensor54 and measured by the microcomputer 74. Based on the total illumination56 and the initial intensity level, the microcomputer 74 then determines96 a corrected light intensity level that will result in the desiredtotal illumination 56 being applied to item 14. Then, the microcomputer74 reapplies 98 to the item 14 supplied light 52 of the correctedintensity level, by producing a light control signal corresponding tothe corrected illumination. Because the sensing of light by theillumination sensor 54 includes sensing any ambient light 58 which isalso illuminating the item 14, this method compensates for the ambientlight 58 by correspondingly reducing the amount of supplied light 52 soas to result in item 14 being illuminated with the desired predeterminedillumination even in the presence of the ambient light 58. Because theintensity of the ambient light level can change, or the amount ofambient light 58 reaching the item 14 can be obstructed, the sensing,determining, and reapplying are repeated at a certain interval (in theabsence of a new predetermined level being specified) so as to maintainthe predetermined illumination regardless of fluctuations in the ambientlight illumination (“No” branch of 99). When a new predeterminedillumination is specified, the method is repeated (“Yes” branch of 99)beginning at 92. While the above implementation of maintaining thepredetermined illumination on the item is performed digitally by amicrocomputer 74, those skilled in the art will recognize that analognegative feedback circuit known in the art can also perform the samefunction.

In some embodiments, the light controller 24 also includes specialeffect inputs 82. Each of these inputs generates a special effectssignal that is detected by the microcomputer 74. In some embodiments,special effects signals can also be transmitted to the programminginterface 66 of the light controller 24 from an external source such asthe vision system controller 16. Special effect programs can be definedin the microcomputer 74 and performed whenever the corresponding specialeffects signal is detected. For example, a signal can be predefined to“flash” the light source 18 by doubling the intensity level on all LEDs36 a,b whenever the signal is received. Another signal might bepredefined to change the color of the lights by alternately illuminatingred LEDs 36 a and green LEDs 36 b each time the signal is received. Or,a signal may be predefined to “ramp” the intensity level on all LEDs 36a,b from minimum to maximum at a certain rate and then return to theprevious illumination settings. By using the appropriate controlprogram, the light controller 24 can be configured to perform other suchuseful special lighting effects upon receipt a special effect signal. Inthe preferred embodiment, each of the special effect inputs hascircuitry which allows a 5 to 30 volt electrical signals to trigger thespecial effect. Such circuitry is both TTL compatible, and compatiblewith the factory floor standards used in the manufacturing environmentsin which vision systems are frequently employed.

From the foregoing it will be appreciated that the lighting arrangementand method provided by the present invention represents a significantadvance in the art. A lighting arrangement can be constructed accordingto the present invention so as to produce a constant light output of aspecified intensity, and of a specified hue and direction, over a longlifetime, and can automatically adjust the light output in the presenceof ambient light in order to maintain a contain illumination on theobject being illuminated. Although several specific embodiments of theinvention have been described and illustrated, the invention is not tobe limited to the specific methods, forms, or arrangements of parts sodescribed and illustrated. In particular, the negative feedbackfunctions described herein can alternatively be performed by anothercomputer attached to the light controller, such as the vision systemcontroller, rather than by a microcomputer in the light controller,resulting in a simpler “dumb” rather than “smart” light controller. Theshape and configuration of the light source can be varied from theembodiments illustrated herein without diverging from the presentinvention. Furthermore, the lighting arrangement described herein is notlimited to applications involving vision systems in manufacturing orquality control applications, but also can advantageously providelighting for other imaging applications, such as close-up photography ordental photography. The invention is limited only by the claims.

What is claimed is:
 1. A lighting arrangement for illuminating an objectto be observed by a vision system, comprising: a light source forgenerating a predetermined light output in response to a light controlsignal; a housing positionable in optical proximity to the object, thelight source disposed in the housing such that the light illuminates theobject during observation by the vision system; and a light controllerin electrical communication with the light source for supplying thelight control signal, the light controller including an input forreceiving from the vision system a command specifying the predeterminedlight output.
 2. The lighting arrangement of claim 1, wherein the lightsource includes a plurality of lighting elements each disposed in acorresponding one of a plurality of locations in the housing.
 3. Thelighting arrangement of claim 2, wherein at least some of the lightingelements are light-emitting diodes.
 4. The lighting arrangement of claim3, wherein the predetermined light output includes a hue, and at leastsome of the light-emitting diodes emit light of essentially the samehue.
 5. The lighting arrangement of claim 3, wherein the predeterminedlight output includes a hue, and at least some of the light-emittingdiodes emit light of a different hue from at least some others of thelight-emitting diodes.
 6. The lighting arrangement of claim 5, whereinthe intensity of light emitted from the at least some light-emittingdiodes is independent of the intensity of light emitted from the atleast some other light-emitting diodes in order to vary the hue of thelight output.
 7. The lighting arrangement of claim 3, wherein at leastsome individual ones of the light-emitting diodes emit light of avariable hue.
 8. The lighting arrangement of claim 3, wherein theintensity of light emitted by each light-emitting diode is controlled bypulse width modulation.
 9. The lighting arrangement of claim 3, whereinthe housing is a toroid having at least one light-transmitting planarsurface, and wherein the light emitting diodes are disposed around thetoroid so as to evenly illuminate the object with light transmittedthrough the light-transmitting planar surface.
 10. The lightingarrangement of claim 9, wherein the light-transmitting planar surfacefurther includes a diffuser mounted between the light emitting diodesand the object.
 11. The lighting arrangement of claim 3, wherein atleast some of the light-emitting diodes emit light of a differentintensity from at least some others of the light-emitting diodes. 12.The lighting arrangement of claim 11, wherein the intensity of lightemitted from the at least some light-emitting diodes is independent ofthe intensity of light emitted from the at least some otherlight-emitting diodes so as to vary the directionality of theillumination on the object.